Expansion tanks are used in hot water heating systems. During usage of the system, the volume of water will expand and contract and the variations in volume are created by the increase and decrease in the temperature of the boiler water as it is heated and cooled in the normal cyclic operation of the heating system.
The conventional expansion tank includes a flexible pressure member or diaphragm which divides the tank into a liquid chamber and a closed air chamber, and the air chamber provides a compressible cushion that compensates for the expansion and contraction of the volume of the water.
It is also known to combine a fill valve with an expansion tank in a hot water heating system, as described in U.S. Pat. No. 3,089,513. In the arrangement described in that patent, the fill valve, which controls the flow of incoming makeup water, is operated by movement of the diaphragm of the expansion tank. As the volume of liquid decreases, the diaphragm will flex outwardly and act to open the fill valve to admit additional quantities of makeup water. Conversely, as the volume of water increases, the diaphragm will flex inwardly, enabling the fill valve to close under the biasing action of a spring.
The fill valve assembly of U.S. Pat. No. 3,089,513 also includes a check valve in the inlet passage which permits the flow of makeup water into the valve assembly, but prevent flow in the opposite direction to thereby eliminate any possibility of the of the water in the heating system contaminating the fresh water supply.